text:
Structural rating of aircraft to withstand storm induced structural stresses
I was watching an accident report where an aircraft crashes when  a storm destroys a wing spar.

I'm curious is this only a danger for small aircraft or are there situations when a storm has caused  structural damage to large commercial aircraft?
Even among smaller aircraft are different models built to withstand different limits of such storm induced structural stresses & what would be the metric to compare them by?

Depends on what weather you mean and how you define "small aircraft". or example, in 1981 a Fokker F-28 crashed due to a tornado: https://aviation-safety.net/database/record.php?id=19811006-0

All aircraft can in theory be overstressed by either a vertical gust (flying into an updraft) that is severe enough or a maneuvering input (pulling on the stick) that is hard enough.  Both increase angle of attack suddenly increasing the lift force, and bending, on the wings.  Increase the lift force enough to exceed the bending strength of the wing's spar, and that's all she wrote.
The protection from this comes from the fact that if you are below a certain speed, you will exceed the stalling AOA before the bending load exceeds the limit, and the stall will partially unload the wing before it gets over stressed.
So there are two speeds you have to observe; Maneuvering Speed and (on larger airplanes) Turbulent Air Penetration Speed (also called Rough Air Speed).   Maneuvering Speed is the speed below which you'll stall before over stressing the wings if you just yank the control column back as hard as you can to full travel.  Instead of pulling the wings off you'll just snap roll (which is in fact, with rudder added, how you do a snap roll).
Turbulent Air Penetration Speed is the speed below which an extreme vertical gust will take the wing past its stall AOA before the gust can exceed the wing's bending strength.
On transport airplanes, if you know you will, or expect there is a significant risk that you will, enter turbulence, you slow the airplane to Turbulent Air Penetration Speed as protection against over stress.  You would do this if there are thunderstorms in the area (you still give storm cells a wide berth in any case), or there are reports of Clear Air Turbulence, or you are crossing the boundary of a jet stream at altitude, where CAT is typically found and you are being cautious.
On the CRJ700, the Turbulent Air Penetration Speed is actually in the lower end of the normal cruise range, 280kt/.75 Mach, so it's not a major inconvenience.  Maneuvering speed is well below that.
Light aircraft like the Piper Saratoga in the video don't usually publish a Turbulent Air Penetration Speed, just a Maneuvering Speed (134kt for the Saratoga), and the pilot should have slowed to that to avoid being over stressed from turbulence.
Problem is, light airplanes bumbling into thunderstorms usually end up in spiral dives where limiting the speed becomes impossible and they typically come out the bottom of the storm in pieces from over speed and/or over stress, and it comes down to staying out of them at all costs.


text:
Testing control surfaces pre flight; what feedback does pilot recieve?
During taxi / pre-take off I see a lot of commercial airliners test the deflections of their various control surfaces like the rudder, ailerons etc.
What's the feedback received in this operation i.e. The pilots themselves cannot visually verify the full deflections achieved on command, right? Unlike on (say) a carrier etc where there's a spotter on the ground to confirm.
Just curious, what is the failure mode to be revealed in this sort of test. Is there some sort of internal avionics indicator that would tell the flight crew if the control surface was not reacting to the inputs as expected?

According to this PDF from Airbus (see section "The importance of the pre-flight, flight controls check"), the PF (pilot flying) moves the sidestick around in a defined manner while the PNF (pilot not flying) watches the ECAM F/CTL (flight control) page and calls out "full up", "full down", etc. (Actually the procedure is described in the A330 SOP but the linked document explains it in a comprehensible fashion.)

Source
The ECAM F/CTL page displays the actual position of the control surfaces. This video from Captain Joe explains the behaviour of the sidestick in an Airbus and shows how the control surfaces can be checked on the ECAM F/CTL page (from approx. 1:40 to 2:15).

Interesting! Somehow I never noticed this set of calls, "full up" "fell left" etc. in any of the flight videos I saw! Thanks for this explanation.

@curious_cat You are welcome. Some of the call outs can be heard in this video from around 6:50 to approximately 7:15. It also shows the ECAM displaying the flight control's positions.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Aircraft with load sensors in their undercarriage to calculate CG
In another question on SE aviation I read that certain aircraft have load sensors in their undercarriage to calculate CG. Which are these types?

The technology to build load sensors into the undercarriage is
  available and it appears that a few planes are equipped with it.

PS. I don't mean just the weight on wheels sensor which is more a switch than a measurement.
Would an accurate weight of a loaded aircraft be helpful?

What make you think a weight on wheel sensor device is just a switch?

@ManuH Oh, I'm sure I am wrong. But would love to be corrected. Thanks!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
List of aircraft which are not certified to be flown by one pilot
Is there a list somewhere, where it says which aircraft are not single pilot certified?
My question is motivated by this question:
Fine for operating not single-pilot certified jet with one pilot

It may be easier to come up with a list of them that are certified single-pilot, and assume that the rest are not. I'd also try to narrow this down, almost all GA light-aircraft (under 12,500lbs) are certified single-pilot.

@Ron Thanks!  I guess I meant aircraft with twin controls that are not allowed to be operated with a single pilot?  Isn't that the shorter list?

@curious_cat Most general aviation aircraft (Pipers, Cessnas, Mooneys, etc.) have dual controls and are certified for single pilot operation. Right off the top of my head, I can’t think of a single-engine turboprop that requires two pilots. When you get into light jets though, you start to see some that require two pilots and some that require only one. Cirrus and Cessna Mustang only require one pilot as do some of the Cessna Citation series.

@JScarry Thanks! That's exactly what I was looking for. But is there a list where this is defined? What's the canonical reference for this? Is there a FAA document that lists this, any idea?

@curious_cat There is no list that I know of. You might want to start a Wikipedia page to list them. Searching for light jet, VLJ, single pilot would get you started. There probably aren’t more than a dozen or so at the moment.

@curious_cat There are also a lot of twin-engine turbo-props that were flown by the airlines as two pilot but are now flown by freight outfits as single pilot. There are a couple of Metroliners that UPS uses on our field and I’d imagine lots of SAAB and Beech 1900 commuters too.

@JScarry So did the certification criterion change when they were sold by the airlines to the freight outfits? Or were they always rated to be flown by single pilot but airlines always flew with two?

@curious_cat Airlines have different rules than Part 91 and freight operations. e.g . §121.385(c) The minimum pilot crew is two pilots. The rules for the freight operations dictate how many pilots are required—subject to the minimum number of pilots in the airplane certification. So the certification of the airplane hasn’t changed, but the rules under which they fly has.

I dont know of any list off hand but the FAA publishes all their type certificates here which do contain the relevant information needed to compile such a list. As mentioned in the comments most small GA planes are single pilot certified so if we start with that assumption you will find a lot of lists that cover the topic of "the largest single pilot certified aircraft" like this one. These lists provide you with the ones that are on the fence size/speed wise.


text:
Making a plane fit to fly after accidental emergency exit / slide deployment
Every so often there is a report of a person openening an ( armed ) emergency exit unauthorised on the ground leading to a slide deployment.
Often this is followed by the flight leaving after deplaning / arresting the person involved but with ( say) an hour's delay.
What i was wondering is how they manage this? Isn't the emergency slide packed in a very specific manner by trained packers? Or do airports carry spare pre-packed slides that they can replace into the doors?
Or are aircraft allowed to fly with a non-operantional single slide as a MEL item?
Just was curious to know since one hour sounded like a pretty quick fix.

Can you link an article where they actually left with only an hour delay? I couldn't find one in a quick search...

I'm no expert by any means, but it might be possible that, if the plane is at a maintenance hub, they may be able to replace the entire door, including a pre-packed slide, in only an hour. Of course, that's just me thinking out loud, and I have no experience or evidence to back this up, hence a comment, not an answer.

Interesting, @Bianfable. Last flight I took (A320 or 321, I believe), there was a large bulge in the main entry door that appeared to have the slide in it. Maybe it depends on which slide is deployed. Maybe I completely misinterpreted what I was looking at.

"The flight" may depart an hour later, but it may be a different aircraft  used for that flight. Replacing the door (big, heavy, multiple hinges needing exact alignment) would be far more involved than replacing the packed slide. Replacing a door in an hour would be like an Indy pit crew replacing 4 tires in 20 seconds... possible, but not going to happen in real life with line mechanics (or your local tire shop).

@FreeMan Actually, looking at escape slide deployment videos (737, A320), it looks like you're right. They are stored in that bulge in the door. I remember seeing some image with a hatch below a door saying CAUTION ESCAPE SLIDE, but I don't remember which aircraft that was...

If you're going there, @RalphJ, it would be like an F1 crew replacing 4 tires in 2.3 seconds. ;)  Fair points, all, thanks for the insight. Many things I hadn't considered, but as noted originally, it was speculation.


text:
Aircraft Emergency Evacuation slide inflation system: Why is a mixture of CO2 & N2 used instead of pure N2?
I was intrigued knowing from this SE-Aviation post that modern, aircraft Emergency Evacuation-Slide inflation-cylinders are often filled with a CO2+N2 mixture.
See  here:
Which gas is used to fill the evacuation slides of modern aircraft?
Intuitively the reason behind the CO2 addition instead of using simply pure N2 is not very obvious to me. Digging deeper into the technology, Wikipedia reveals that "The CO2 is used to slow down the rate at which the valve expends the gases."
Any idea about the science behind this? Why would CO2 slow down the inflation rate? And why do we want to slow the inflation in the first place? Fabric damage?
Also, what is the CO2 percent in the mix, anyone know?

Let me first give some background information on the art of inflating an evacuation slide. There are two ways to fill them:

Expand gas from a high-pressure bottle: This will cool the gas as it expands, so ice may form at the aspirator outlets where sucked-in air and the expanding gas mix, blocking the flow of outside air into the slide. Also, storing compressed gas needs bulky and heavy steel vessels.
Pyrotechnic gas generators which use an exothermic reaction to create a large amount of hot gas. They are more compact and lighter than a pressure vessel for the same amount of gas produced, but their hot gas could scorch the slide material and will contract when it cools down after the filling process, so a slide filled by a pyrotechnic gas generator will sag after a while.

Because of the disadvantages of both pure stored gas and pure pyrotechnic inflation devices, current state of the art emergency evacuation slide systems use a hybrid inflator, which comprises a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel.
Operation of a hybrid inflator
The typical gas generator uses solid ammonium nitrate which is electrically ignited and produces nitrous oxide (N$_2$O) and water (H$_2$O) when heated. Nitrous oxide will exothermically decompose into nitrogen and oxygen when heated above 650°C or 1200°F, so even more energy and gas is produced. Note that the decomposition of two moles of nitrous oxide will produce two moles of nitrogen (N$_2$) plus one mole of oxygen (O$_2$), thus adding 50% more gas volume. This hot mixture of nitrogen, oxygen and steam is then piped into the pressure vessel and mixes with the gas flowing out from it, thereby heating the emitted gas.
The pressure vessel contains gasses like nitrogen or carbon dioxide. Other candidates are argon or helium, but their higher price has held them back. To increase the yield from the pressurized gas, the trick with the nitrous oxide can be repeated here: By storing nitrous oxide in the pressure vessel, again 50% more gas volume can be produced from what is stored, while the exothermic decomposition produces additional heat that offsets, to some extent, the heating of the stored gas.
Nitrous oxide is also attractive because it offers the best manufacturability, storage and handling of the possible decomposable gas source materials. Although classified as an oxidizer, in practice it is generally nontoxic and non-corrosive and is relatively inert up to temperatures of about 200°C. Also, nitrous oxide, as compared to gases such as oxygen, nitrogen and argon, liquefies relatively easily at ambient temperatures - helpful when the gas in the pressure vessel is stored in liquefied form to reduce its bulk.
Heated to its decomposition temperature, pure nitrous oxide will undergo a rapid disassociation chain reaction unless a coolant gas is added to absorb some of the heat generated by the exothermic reaction. Therefore, the mixture in the pressure vessel contains 50% or more of carbon dioxide which has a latent heat of vaporization of approximately 66 Calories per gram. As a three-atomic gas it is better suited for this task than two-atomic alternatives. Thus, it does not serve as a diluent (as aeroalias wrongly states), it absorbs heat and stabilizes the disassociation process.
Why a mixture of nitrogen and carbon dioxide
Now we can answer your question: Nitrogen is contributed both by the pyrotechnic gas generator and by the compressed gas and is produced from the dissociation of nitrous oxide (N$_2$O). Carbon dioxide is added to dampen the exothermic dissociation and absorb some of the heat produced. Without it, the gas generator would experience a runaway chain reaction! Its concentration in the compressed gas is between 50% and 75%, so the concentration in the gas generator mix is around 35% to 50%.
The high speed flow of gas from the pressure vessel is now piped through an aspirator, which sucks in ambient air by using the Venturi effect, and from there into the slide. This contributes up to three quarters of the total gas in the slide, so most of the nitrogen in the slide is from ambient air. In the slide, the concentration of carbon dioxide is diluted to around 10% to 15%.

Aspirator (from BF Goodrich's US Patent 4368009). High-speed gas from the gas generator (red arrows) enters from the angled pipe on the left and pulls ambient air (blue arrows) with it when shooting from the nozzles on the right. Contrary to aeroalia's claims, an aspirator is not just another name for a gas generator, it is an entirely different thing altogether.

Excellent answer, thanks! So much complexity hides in the most unexpected places. :)

As an aside, why not use the typical automotive airbag inflation mix? I recall that's a  mixture of sodium azide (NaN3), KNO3, and SiO2?

So prior to activation what is the physical state of the CO2 in the system? Liquid? Soild?

@curious_cat sodium azide decomposes explosively. That's perfect for an airbag which inflates then immediately deflates all in under a second, but not what you want for an escape slide that inflates over the course of several seconds and stays inflated for many minutes. Also, sodium azide in the quantities required for an escape slide would be downright dangerous.

@curious_cat: Normally gaseous, but the latest innovation is to use partly liquified storage to make the bottle smaller. No solid storage, because ammonium nitrate does not give off carbon dioxide and is the only solid source of gas in this process. Regarding complexity: The first evacuation slides more than 50 years ago simply used compressed air, until people found that storing oxygen at high pressure might not be such a brilliant idea.

@PeterKämpf Storing air at high pressure might not be brilliant but Nitrogen seems safe enough. Only downside is the weight of a pressure vessel I suppose.

@PeterKämpf Just to clarify my understanding: If the CO2 stored is normally gaseous then how does the  latent heat of vaporization that you've mentioned in your answer matter? CO2 would moderate the reactive exotherm via the latent heat mode only if it were solid / liquid pre-inflation, correct?

@curious_cat You're correct about the gaseous nitrogen. Storing it as  a compressed gas is safe, but needs a heavy pressure bottle. Now to nitrous oxide: If heated above 650°C it will exothermically split into nitrogen and oxygen, and it is this extra heat which needs to be kept in check by the carbon dioxide. To get away with smaller bottles, the most recent systems store both gasses in liquid state and pressurize the vessel with argon or nitrogen.

@PeterKämpf My point was the using CO2 to moderate the exotherm only makes sense if the CO2 is stored as liquid or solid under pressure.  Otherwise the heat moderation benefit offered by CO2 as gas is identical to that offered by gaseous N2. Do we agree on this? i.e. Since you mentioned the "Latent Heat" of CO2 that comes into play ONLY if CO2 is stored as a Liq. Right?

@PeterKämpf They should have tried pressurized Hydrogen. It heats when it expands through a nozzle.  No ice crystals problem. Just kidding of course, but it'd sure solve the ice crystallization problems in the aspirator!

@curious_cat ... and the slide would float better when used as a raft. But as a gas it should also cool when expanded - it is a two-atomic gas and behaves like all other gasses. Note the difference in the specific heats between CO$_2$ and N$_2$ on the linked page, though. They do have their own properties, but I still struggle myself to fully comprehend the reason for selecting CO$_2$ over N$_2$ as a heat sink.

@PeterKämpf About H2 also cooling when expanding, I think you are wrong. Not all gases cool when expanding (I hope I'm not being condescending by writing stuff you already know! ). Most gases do cool when expanding but this is only because they are below their Joule Thomson inversion temperature.  Very few gases have their Joule Thomson inversion Temperature below Room Temperature. Hydrogen is one of those. So unless, you were expanding hydrogen through a nozzle at cryogenic temperatures in my opinion it would heat up when expanding. I could be wrong. Would love to hear your opinion.

@PeterKämpf Of course, I'm implicitly assuming that the nozzle in a evacuation gas pressure vessel leads to  a Joule Thomson expansion i.e. an isenthalpic expansion. If it were an isentropic expansion then you would be correct: even Hydrogen would cool. But I think it is a good assumption to think of our example as isenthalpic and not isentropic. Do you agree?

@curious_cat: Uhh - isenthalpic sounds like no change in the kinetic energy, which means the pressure is converted into frictional heat. That leaves too little energy for the aspirator to do its job, and now you need to have three to four times as much gas to fill the slide. Using hydrogen for this will need a laaaarge bottle ...

@PeterKämpf Indeed. I think you are right. I was mentally modelling the aspirator as a throttling valve. In hindsight it seems more like a nozzle / steam ejector. So, yes, even hydrogen would cool in an isentropic expansion. :)

@PeterKämpf I was reading the link you sent about specific heats. It shows CO2 as 0.84 versus N2 as 1.04. In that case, wouldn't N2 be a better heat sink than CO2? What gives?

@curious_cat: I'm still struggling to understand it myself. Maybe the reason is that carbon dioxide cools less in an adiabatic expansion than do two-atomic gasses.

@PeterKämpf One thought: We are looking at per kg specific heats. We should be comparing per volume specific heats which are essentiall comparing per mole specific heats. If you do that CO2 has a higher specific heat than N2. So adding CO2 helps more than N2. Does that make sense?

@curious_cat: Yes, it does make sense if volume is restricted and the increased mass per heat capacity can be accepted. Since the slide canister on the inside of the door should indeed be as small as possible, your reasoning is sound.

Why not use gas stored in a heated high-pressure bottle, to counteract adiabatic cooling?

@Sean: Do you want to wait until the bottle is heated (while flames might consume the interior of the cabin), or do you want to keep the bottle permanently heated (and make it a fire hazard)? No, I'm afraid that heating the gas bottle first is not a good way to counteract adiabatic cooling - unless you use a gas generator as current slides do already and mix hot gas to what is in the bottle already.

The $\text{CO}_{2}$ is used as a diluent.

The pressure vessels (for inflating the slides) usually have nitrous oxide (or similar) compounds, that undergo exothermal thermal decomposition during inflation. For example, $\text{NO}$ reaches temperatures of $\sim 650\,^{\circ}\text{C}$ during thermal decomposition. As $\text{CO}_{2}$ absorbs heat during its expansion, this prevents any damage to the fabric.

For the same reason, the presence of $\text{CO}_{2}$ controls the reaction rate of the decomposition of the nitrous oxide as it is usually impossible to prevent decomposition from generating unpredictable amounts of gas.

The $\text{CO}_{2}$ forms around 50% to 75% of the mixture.

The amount of gas required for inflation of the slides in the aircraft is quite high. If all these are carried in pressure vessels, either of the following has to be done

increase the pressure (already $\sim200$ bar) of the gas in the vessel (its usually in liquid form), which increases the pressure vessel thickness and consequently the weight, or

increase the number or size of pressure vessels, which again increases the weight.

Also, a gas generator (usually pyrotechnic) can be used, which generates gas by exothermic reaction. This too, has its disadvantages:

The gas generated is at high temperature (via thermionic reaction), resulting in damage to the slide fabric.

Also, as the gas cools, the evacuation slide may sag (as gas contracts). This is not good as the slides are expected to act as life rafts.

The size of the gas bottle can also be reduced by using an aspirator, which draws in ambient air to mix in a ratio of about two-thirds air to one-third gas from the bottle.
To overcome these, the aircraft slide inflation system uses is hybrid system that uses two techniques:

Instead of carrying the gas itself, carry a compound capable of undergoing thermal decomposition such that the number moles of gas produced by the inflator is greater than the number of moles of gas stored. Nitrous oxide is used because of manufacturability, storage and handling concerns.
Two moles of Nitrous Oxide $\text{N}_{2}\text{O}$, when undergoing thermal decomposition, forms two moles of diatomic nitrogen ($\text{N}_{2}$) and one mole of diatomic oxygen ($\text{O}_{2}$)

$2\text{N}_{2}\text{O} \ = \ 2\text{N}_{2} \ + \ \text{O}_{2}$
Because the universal gas constant is the same for all gases, the three moles of nitrogen and oxygen produced by the decomposition of two moles of nitrous oxide occupy 50% more volume than the two moles of nitrous oxide would have occupied at the same temperature and pressure. Thus, the size of the container is reduced by a third (compared to situation where we use undecomposed gas).

A pyrotechnic gas generator (or an aspirator) that augments the stored compressed gas by providing additional gas.

For more details, see US patent application US 6877698 B2, Aircraft evacuation slide inflation system using a stored liquified gas capable of thermal decomposition

In case the amount of gas required is low (for example, in some helicopters), only the pressure bottles are carried.

Interesting. So, this mixture you describe seems totally different from the one described on Wikipedia as well as the other SE-Aviation Thread? That one had N2 in it & no NO.

As an aside, why the NO & exothermic decomposition? Why won't a simple compressed gas do? e.g. compressed N2

@curious_cat I've edited the answer as I thought its better to answer them there. Have a look.

Wait what? I thought that 2NO2 = N2 + 2O2?

@Zizouz212 I made a typo. Nitrous oxide is N2O, not NO2. Corrected it. Thanks for catching.

Ah, okay. I began to think that my chemistry course is a lie xD

No, it is not used to dilute anything. Please try to read and understand the patents before you cite them. Your answer is full of factual mistakes.

@curious_cat: Expanding a compressed gas will drastically cool it. You'd get frostbite - not good either. A careful mix like this will balance the heating from decomposition with the cooling from expansion.

@MSalters Does anyone care about frostbite? Who's touching the valve? The one reason I'd care about is if I was aspirating external air then I'd worry about moisture freezing up & choking the ports.

@curious_cat: The gas itself is that cold too. And yes, you are in fact pulling in moist external air as well. (See Peter kampf's answer). Ice would clog up the aspirator. But I now realize that there's an even bigger risk. When you cool down the inside of the slide to -40 or so, the material becomes brittle.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How much does a drag chute system weight?
How much weight does a drag chute add to a plane?
Let's take the Tu-114 as an example. This plane has a loaded weight of 68.1 tons (I could not find a max landing weight figure). I could not find how much it's drag chute system weighs.
One purpose of this question is to compare it to thrust reverser systems. I have a separate question on that.

Interesting comparison. However, remember that TRs stow themselves back into the nacelle after use. Drag chutes are discarded on the runway and need to be collected, repacked and reloaded on the aircraft. This fouls the runway for the next aircraft. They could be disconnected on the taxiway, but then they remain as FOD for the following craft.

I see where you're headed... :)

@FreeMan Never knew that about discarding chutes on the runway. Yeah that seems unnecessarily...what's the word...unsophisticated? It makes me wonder if some tiny wind-up motor can reel the chute back in, then at the gate some rampies can repack it properly.

Drag chutes were basically designed for military use where you have lots of Private power to handle things like this. Applying the use of chutes to civilian, commercial aircraft use would take a lot of redesign, rethinking, and plenty of spares because you'd have to replace the "chute pack" between each flight and send the expended one off to be inspected & repacked. Plus, can you imagine all the foreheads smacking against the seat back when the chute is deployed? hmmm, makes me want to see this happen just for the entertainment value!

It depends on the aircraft- for example, the the brake parachute system of the 17t F-16 weighs 20 lbs.
Generally, in an aircraft the brake chute system will weigh much less compared to the thrust reverser. While I'm not able to get any 'official values', according to this thres,

In a Lear (24-25) the drag chute weighs some 25 lbs, the Dee Howard reversers weigh 350 lbs...

20 pounds? Are you sure that's not the pilot's parachute that deploys after ejecting?

@Drz214, the link provided by aeroalias clearly refers to the drag chute.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What was the takeoff run distance of a Fw 200 Condor bomber/transport?
The Fw 200 Condor was a large plane of Germany during WW2. It was used in recon and bomber roles against convoys. Wikipedia lists several specs including max takeoff weight of 24.5 tons and 4 engines producing 895 kW each.
But I could not find takeoff run distance, neither there nor in google. I would like to know how long of a runway was needed for it.

Unfortunately i could not find this information for the basic variants, but for the Fw200F an Focke Wulf document from 1943 states following
on a paved runway, 4x 323R engines on standard start power
********** (deleted / see graph)

on max power
   ********** (deleted / see graph)

EDIT:

the document is called  "Focke Wulf Fw 200 F Fernaufklärer mit erhöter Reichweite" page 29  http://www.germanluftwaffe.com/archiv/Dokumente/ABC/f/FockeWulf/Fw%20200/Focke%20Wulf%20Fw%20200%20F.pdf

ymb1: i think you are right, i got the scale wrong, another thing is the SR vs S20

@alfetta Yeah I'm curious: what does SR and S20 mean?

just a guess S20 could mean, run distance till reaching 20m flight height
, this terms i found in an Cessna Handbuch  "Startrollstrecke"" and  "Startstrecke über 20m Hindernis"
.

@ymb1: yes, S20 looks to be the distance to reach 20m height, so with 1000hp / 25t some 1100m and 1050m with 1200hp // and with 1000hp / 25t some 800m for lifting of ground. we should remind, this document is a what if by Focke Wulf and the sheet is not based on some test at Reichlin AB

i found another document, the Fw 200 C-1 C-2 Kurzbetriebsanleitung.Teil 2.-Focke Wulf Flugzeugbau GmbH, on page II 06 the chapter FLUGBETRIEB describes how to start the plane and states with 21.300kg weight it needs 670m for lift off and 1550m for reaching 20m height


text:
Does radar altimeter work over the ocean?
I'm wondering if radio waves are reflected by the sea. If so then the usual radar altimeter would work. If not, then is there any other fallback besides barometric altimeter?
According to the wiki article, for some reason radar altimeter uses radio waves (instead of microwaves like usual radar). This is why I put in the radio tag.

@Airsick according to the tradition EM spectrum divisions, microwaves and radio waves are separate: https://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg

@DrZ214 Which is the problem with over simplistic diagrams. In reality, these boundaries are fairly undefined and microwaves will fall under the category of radio communications for most use cases.

Note that RADAR is an acronym for "RAdio Detection And Ranging" - so it is no surprise that radar uses radio waves.

Yes.  In the Navy, our primary source of altitude indication under 5,000ft is the radar altimeter.  The AGL over the ocean conveniently also happens to be the MSL as well.

How does AGL always equal MSL, given that the ocean has tides?


text:
What was the first supersonic wind tunnel?
What was the world's first supersonic wind tunnel? Who built it and how did it work?
All I've found so far is this Wikipedia article that says:

By the end of the war, Germany had at least three different supersonic wind tunnels, with one capable of Mach 4.4 (heated) airflows.

The citation in the article is a dead link.

The German tunnels were in Aachen (1936, 0.1x0.1 m), Göttingen (1938, 0.11x0.13m) and  Peenemünde (1939, 0.4x0.4m, Mach 4.4).

@mns Thanks but do you have a source so I can look them up?

I knew about the locations from different sources (Bölkow's "Erinnerungen", some googling...). For the years and cross-sections I looked in the same book as aeroalias answer. However, I'm surprised that the tunnel in Göttingen was built so late: reading the phrase before aeroalias citation ("The first suggestion to build a supersonic wind tunnel according to the vacuum-storage principle was made by Ludwig Prandtl in 1917. [...] however, the tunnel could not be realized until the end of World War One. During the same year...") I'd have though that it was built in the early 20s too.

The world's first supersonic wind tunnel started operating in National Physical Laboratory in Teddington, England in 1922.
It basically used a pressure tank upstream, which was used to generate supersonic flow in a cross section of 20.3 mm, which was used to study projectiles.
From the book Aeronautical Research in Germany: From Lilienthal until Today:

During the same year, the National Physical Laboratory (NPL) in Teddington/England started operating, for the first time ever, a supersonic wind tunnel based on the pressure storage principle ...


text:
Why do jet engine igniters require huge voltages?
Wikipedia's Components of Jet Engines has this to say on the igniters:

A high voltage spark is used to ignite the gases. The voltage is stored up from a low voltage (usually 28 V DC) supply provided by the aircraft batteries. It builds up to the right value in the ignition exciters (similar to automotive ignition coils) and is then released as a high energy spark. ...

Most modern ignition systems provide enough energy (20–40 kV) to be a lethal hazard should a person be in contact with the electrical lead when the system is activated, so team communication is vital when working on these systems.

I bolded that part I'm interested in. 20 to 40 kV? That is a huge, huge amount of voltage. Why on Earth would a jet engine need that much voltage to produce a spark? I thought kerosene was relatively easy to ignite.

Whoever wrote "energy (20-40 kV)" does not know what energy is.

In short, 40 kV isn't that much voltage for applications that are intentionally creating an electrical arc. Car spark plugs also use voltages in the tens of thousands of volts for the same reason, for example.
As for why that is:
In general, air acts as an electrical insulator. That is, electricity won't pass through air at normal voltages. Which is good because otherwise you'd have a constant arc to ground through the air from any exposed hot conductor and that would cause lots of problems.
As with any insulator, air has a dielectric strength, measured in volts per unit distance. For a given length of air (such as the spark gap in a spark plug,) there exists a breakdown voltage at which point the voltage differential per unit distance exceeds the dielectric strength. When a voltage equal to or greater than the breakdown voltage is applied to conductors on either side of said length of air, the air will suddenly ionize, creating a channel of plasma through the air. The plasma channel now has a much, much, much lower resistance than air normally has, allowing current to flow across the gap and quickly equalizing the potential (i.e. voltage) on each side of the gap.
As you're likely aware, plasma tends to be quite hot. The plasma channel created by the above process is the visible arc that you'll see between the conductors in a spark plug.
For air at normal pressures, the dielectric strength is about 3 MV/m. That is, if you want to create a 1-meter arc, you'd need 3 megavolts of potential across it. Or, in units more suited to creating ignition sparks (which are hopefully not a meter in length,) you need around 3 kV per millimeter. So, for example, in the winter when you take off a fleece coat and then experience a shock the next time you get close to a grounded conductor, the static potential that has built up on you is about 3 kV per millimeter (i.e. 30 kV/cm or around 75 kV/inch) of the maximum length of the arc you can create to such a conductor.
So, in summary, 40 kV really isn't all that much when you're trying to create arcs through air, especially not highly pressurized air, such as in a piston or jet engine. Even relatively small distances require quite high voltages in order to arc. Which is a good thing so that exposed conductors don't just constantly create arcs to ground. Or to grounded people, electronics, etc.
By the way, this ability of an electric field to ionize air is why switches for very high voltage transmission lines have to have much, much longer poles than what you'll find on normal switches. On a 138 kV line, for example, the electricity would simply arc across a switch that created a gap of less than couple of inches or so when open. Perhaps an even larger gap when humidity is high. And these switches do indeed arc while opening and closing. They can also create quite long arcs while opening, since the plasma channel is already in place. Here's an example of what that can look like at 138 kV:

Of course, in this case, you have a more-or-less continuous voltage source, not just a stored-up charge that is suddenly released, so the arc won't just be a brief flash, but will continue until the distance between the conductors becomes too high to maintain the arc.
And, of course, when the potential between the base of a cloud and the ground gets too high, you get lightning. And, yes, that's a very, very high voltage to arc across a kilometer or more. According to the U.S. National Weather Service, a typical lightning bolt involves potentials of around 300 MV. While that actually seems a bit low to arc across the typical distance of a lightning bolt, lightning normally occurs in thunderstorms where the atmosphere is already very humid and, thus, more conductive than normal.

As far as the ease of igniting jet fuel, while that's mostly unrelated to the voltage required to create a spark across a given distance of air, it's actually not that easy to ignite. At standard air pressure, you can drop a lit match in a bucket of jet fuel and it will just extinguish the match. You can also create a raging fire in the fuselage of a Boeing 777 and the fuel remaining in the tanks of said 777 won't burn. This is by design and it's one of the great things about jet fuel for aviation. While the energy density and specific energy of petroleum fuels is very high (by chemical reaction standards,) it actually requires quite high temperatures and/or pressures to ignite jet fuel, making it an ideal fuel for providing lots of energy per unit weight while not tending to randomly burn or explode at inconvenient times.

This is the best answer here, even briefly addressing the relative difficulty of igniting jet fuel. Thank you.

I feel this answer is somewhat confusing.  It suggests that the spark needed in Aviation engine use is somehow extreme.  the answer that simply points out it';s the normal voltage in any spark system, is clearer.

@Fattie Hmm... the answer states, "So, in summary, 40 kV really isn't all that much when you're trying to create arcs through air, especially not highly pressurized air, such as in a piston or jet engine." Do you think it would be less confusing without the transmission line and lightning comparisons? I just added those because I thought they'd be useful as a point of comparison for voltages required to arc across air gaps.

@Fattie It's true that the voltage needed to arc across any air gap is a bit extreme compared to voltages normally used on wires (because we normally don't want conductors to be able to arc across air gaps,) but 40 kV certainly isn't unusual for applications where you actually do want an arc.

@reirab - I'm all for incredibly simple answers.  I would have said up front "30k is the norm for all sparks, such as in a car."  That's the whole answer to this question per se.  Then - by all means - if I was able to write an answer this good I would then say "Here's some more info on sparks." And then had the answer as seen.

@Fattie Fair point. Added brief summary at the beginning.

@reirab - don't get me wrong. Every writer should follow their own lights.  But then, every  a-hole can criticize from a sofa in comments :) :)

"(i.e. 30 kV/cm or almost 10 kV/inch)" - is it 100 kV/inch?

@EugeneRyabtsev it should be 76 kV/inch, so yes it's not correct.

@EugeneRyabtsev Good catch, thanks. Fixed.

"especially not highly pressurized air" is highly counterintuitive to me, considering the limits at either vacuum or no-longer-gas (at least, I think this is easier to arc than air). Maybe link Paschen's law or something?

@obscurans Good point. Link added.

Great explanation!

That's nothing special.  Car ignition coils also put out 40kV (try grabbing a spark plug wire on your car, that is leaking spark due to insulation breakdown, with your hand while it's running; FUN FUN FUN!).  You need that voltage to jump an air gap reliably in a piston, or jet engine's, combustion chamber.  That high a voltage allows fairly large air gaps to be used, making the spark large and hot, all good things to have when you are waiting for the engine to relight after flaming out due to water ingestion while flying in heavy rain.
The 40kV coils in cars are the reason auto plugs operate with very large gaps, like .040", and will run fine until the electrodes are down to little nubs.

I had no idea cars also needed that. But to jump an air gap...I mean the whole reason for an inerting system in fuel tanks is so that a frayed wire cannot ignite the vapors. Im pretty sure the electrical mains in the wings and fuselage do not run at 40 kV and yet there could be a spark. Maybe its a q for physics.SE but i cannot understand why in one place its so easy but in another place its so hard.

@DrZ214 From this answer on Physics.SE: you need at least 10kV to create a 1cm spark in air. If you want the spark to be produced reliably, more voltage is better.

@DrZ214 don't forget a frayed wire could vibrate and form a small arc repeatedly.  If each arc has a minuscule chance of igniting vapours, and there are very many arcs, the chances of igniting a vapour-air mixture become significant.  As the consequences are extremely severe, the likelihood must be suppressed to extremely low.  It would also be more likely as the tank empties - the headspace over a nearly full tank of most fuels is too rich to ignite, but once some air has been drawn in it's a different matter

@DrZ214 Its the difference between potentially and reliably. An inerting system is about reducing the minimal chance of arc, that would still happen at least a couple of times in a million hours of flights. And a couple of times is too much for the risk of fire. Where as in an ignitor, having an ignitor that only worked 2 times in a million would be unacceptable, so you need a voltage with an extremely high likelihood of arcing.

@DrZ214 one thing to keep in mind is pressure.  The insulating quality of the air increases with pressure, so in a piston engine's combustion chamber or a jet engine's burner can, the high pressures require a higher voltage for a given gap.  The opposite case is a problem for high voltage components in ambient air when you go to altitutude.  Magnetos at high altitude have a problem of high voltage current internal leakage and have to either have large internal clearances to make large air gaps between current carrying parts, or if it's still a problem, the mags are actually pressurized.

@DrZ214 The reason for inerting the fuel tanks is if the insulation on the wires comes off and the conductors actually touch. That will produce quite a nice spark. It won't be able to arc between the conductors at any meaningfully-non-zero distance at those voltages, but that doesn't mean it won't produce quite the spark if they touch. I'm not sure if you've ever seen what happens when a 120 or 240 VAC hot wire touches a ground/neutral wire, but it produces a quite large and extremely hot spark for the hopefully very short duration until the circuit breaker trips.

@DrZ214 Car batteries are only 12-13 VDC and you can make quite nice sparks with those, too. A short will produce enough current to create quite a spark and/or a lot of heat in your wires at just about any voltage.

@DrZ214: You don't run the 40 kV on conductors all around the plane, you have something like an ignition coil or magneto mounted on or near the engine.  That boosts the 12/24 volts of the electrical system to the much higher voltage needed for the spark plug.

Airplanes have lots of 115 vac running every which way often out through the wings for things like heater blankets, and most of the electronics run on 115v 400hZ AC power. The dominant electrical source on airliners is 115 v fixed frequency AC, produced by constant-speed ac generators. DC 28v is secondary for things like emergency services and starting the APU.   Smaller aircraft like turboprops are the other way around, mostly 28vdc, with fixed frequency 400 hZ 110 VAC for avionics (using inverters), and variable frequency ac (from variable frequency generators) for things like heaters.

@JohnK The insulating quality of the air increases with pressure I think it's actually density, not pressure. Correct me if I'm wrong. But I get your point.

Well it's altitude dependent not temperature dependent I think.  Not really sure now.

I appreciate the levity, but obviously readers should NOT try grabbing live ignition parts while the car is running. :) Also, it's better not to let the spark plug electrodes wear down to little nubs, as this will increase the required spark voltage and stress your (far more expensive) ignition coil(s), possibly to the point of misfires, rogue arcing and failure.

@screwtop: And badly worn plugs will make your car (and presumably your piston aviation engine) run less efficiently than it otherwise might.

Well obviously. Just pointing out that they will still work while severely degraded.  Now, the plugs in my Lyc O-290 have to be gapped at not more than .019" because the mags make, maybe, 15-20k and anything larger will overheat them.

40kV is typical for a spark igniter. Air, especially dry air, is a pretty good insulator, and it takes that level of voltage to cause a breakdown. An ignition spark is effectively a miniature lightning strike and propagates through air in much the same way; consequently it needs much the same electrical field strength (in volts per metre) to get started.
But it is wrong to talk of the kV level as "energy". It is more akin to pressure. Static electricity can build up to these voltage levels, but if you walk across a plastic floor in plastic shoes you will not die when you grasp a metal door handle and zap yourself. The energy of a discharge manifests as the current times the time it lasts, and the current from such a simple static discharge is very short-lived.
An ignition system suitable for starting a big engine will be designed to reliably produce a nice fat spark - that is to say, a higher current for longer - and is therefore that much more dangerous.

Large trucks with compression-ignition engines will usually use a glow plug to start, not a spark plug.

@forest True. Large petrol (gasoline) engines do exist but are not common. I have rephrased my answer accordingly.

If one passes current through a coil at low voltage and disconnects it suddenly, it will produce a voltage which is orders of magnitude higher unless current starts flowing again (the only reason voltage is limited at all is that if it gets high enough, current will start flowing again, somewhere).  If one uses two inter-wound coils with different numbers of turns and interrupts a current in the coil with fewer turns, the coil with the larger number of turns will reach a higher voltage yet unless or until current starts flowing through one of the coils.  It is thus simple to have a small assembly turn a low voltage into spurts of high voltage very near the location where the high voltage is needed.  Further, it's possible to use a moving magnet in the vicinity of the coil to induce the initial current in the coil without having to deliver any sort of voltage to it.  This is the principle upon which magnetos work.
Note that such devices do not continuously produce high voltage, but instead store up some energy when current is driven at a low voltage (or using a moving magnet), and then release it all quickly at high voltage.  Devices may pose danger if the amount of energy that they store at once is sufficiently large, if they cycle at a fast enough rate to deliver a substantial average current, or if they cycle it at a rate which can interfere with nerve impulses in the body.
I don't think the text is intended to suggest that the ignition systems could be used as a source of "reliably lethal" voltage, but rather to warn that even inadvertent incidental contact could, under just the "right" circumstances, be dangerous.  Even if the risk of death from an accidental shock would only be 1 in 10,000, a prudent person should not take such risks needlessly.

While this is useful information, it doesn't answer the question at all.

You could add to this by prepending to this answer that the voltage is what it is because it couldn't be more, or less. The voltage will keep building in the ignition coil until it's able to bridge the gap between the spark plug, after which it discharges immediately.

@forest: My point was that while 30,000VDC may sound like a lot, or would pose an ignition danger, or it would be hard to produce, it's fairly easy to produce, and the dangers can be minimized by generating such voltages near the point of use.


text:
How much weight does a blisk save?
A Blisk is a bladed-disk, all one monocoque component.

So the blades do not have to be inserted and secured, and they are not removable. No nuts and bolts either.
The downside is that any serious damage to one blade, and the whole blisk must be replaced. The upside, I have heard, is that they save weight. Wikipedia says "efficiency improvements of up to 8% are possible." but I'm not interested in aerodynamic efficiency right now. I'm interested in weight savings.
So the question is, how much weight can you save? Is it really significant? Interested in compressor and turbine blisks and I would not be surprised if the answer varies between them.


text:
Why doesn't the A320 have a dorsal fin like the 737?
The A320 doesn't have a dorsal fin, but the 737 does. If the 737 is more or less a similar aircraft, why doesn't the A320 have a similar dorsal fin?
By Bill Larkins [CC BY-SA 2.0], via Wikimedia Commons

In general, see dorsals and strakes

Adding a dorsal fin, increases surface area. The plus theoretical side is that it increases directional stability. The minus theoretical side is that it adds weight, drag, and manufacturing complexity.
Early 737s did not have a dorsal fin. Presumably, at some point Boeing decided that adding one would provide better performance.
Airbus apparently does not see the need for on in the A320.
They are all design decisions.

not performance but safety is improved. If you have a bad record it pays to go to some extra effort.

With due respect, I don't see how this post can be an answer to the question, it's mostly speculation (theoretical, presumably, apparently, ...), references are needed. There is a much better alternative.

Boeing learned the value of a dorsal strake the hard way: On their civilian version of the B-17, the Boeing 307 Stratoliner, the first Boeing aircraft with a pressurized cabin, the lack of a strake ahead of the fin caused the rudder to lock in the hard-over position when the pilot demonstrated the capability of the aircraft to fly with two dead engines on one side. The aircraft entered a spin and crashed, and that was on a demonstration flight for prospective customers.

Early Boeing 307 with the two right engines off and props feathered (picture source). Note the deflected rudder and the rectangular windows in a pressurized fuselage. It worked because the pressure difference was small.

Late Boeing 307 in flight (picture source). The vertical was enlarged and had the strake added; the same vertical was also used on later versions of the B-17.
With today's hydraulically operated controls the risk of an uncontrollable hard-over is reduced, but exactly this happened to several early Boeing 737s. From the 737-300 on the strake was added and the control system (PCU = power control unit) was exchanged for a  modified version.
Airbus never had those issues and did not see the need to add a strake since the existing vertical could do its job in all conditions. A strake is worthless at small sideslip angles and will only help when the vertical is close to stalling due to extreme sideslip. Adding a strake will increase the wetted surface area and add friction drag, so Airbus did not include one.

One of the rudder hardovers was a -200 but the other was a -300 which already had the strake.

There were many incidents with the 200, and the PCU was the major culprit. Adding the strake was a precaution.

@ymb1: Could be, my precaution comment was meant for the hard-over accidents. More thrust will cause more sideslip in one engine out cases.

@PeterKämpf Why did the lack of a strake cause the rudder hardover? I'm having trouble connecting the two

@Ksery: Both are not connected!

It just indicates a cleaner design on Airbus' part.  Dorsal strakes, wing fences, vortex generators etc are just bandaids for poor airplane design.

@PeterKämpf But you literally said "the lack of a strake ahead of the fin caused the rudder to lock in the hard-over position ". I think I'm missing something crucial here

@Ksery: Now I understand, you were not referring to the 737 but the 307.  Yes, I did. This happened on the Boeing 307, and I have witnessed the same on other, smaller airplanes where control forces were still manageable with a hard-over rudder. The rudder will stall at an earlier sideslip angle and the suction of the separated flow will cause the hard over rudder. Adding a strake will cause a vortex which will mix the flow on the leeward side so the separation does not occur as early. See here for vortex lift

@PeterKämpf Thanks for the clarification, and wow, that is quite a scary phenomenon

In addition to ymb1's answer, I found this video of a B737 Pilot that specifically explains why is added in new models.
The short answer is very similar to that already provided, but explained in details and with some examples, maybe could result easier for someone to figure out reasons.
Boeing had two options "to cope with greater asymmetric thrust potential" increase tail height or enlarge its surface horizontally, but existing customers preferred second option because they already had hangars designed for previous B737 series with not enough height.
Continuing comparison to A320 family, we can also say that in A318 (the shortest of the family), Airbus too needed to enlarge tail surface (increasing its height in this case) because shorter fuselage caused less torque that should be compensated by additional force at the end.
For those that are more technicians, physical explanation could be the following:
Since the torque on a body is defined as T=F x d, where "F" is the applied force at a point P of the body, "d" is the distance from P to the center of the rotation and "x" is the cross product, it's clear that by reducing d we need to proportionally increase F to keep T constant

The extreme examples are the A300-600ST Beluga and the new A330 Beluga XL vs the normal versions.

Note in your photos that the A320's tail in general is wider than the 737's.
So it can be argued that Airbus met the same need with a different tail design. They didn't use a strake, they made the whole vertical stabilizer larger.
The fact that the 737 was originally designed in the 1960's for turbojet engines, while the A320 was designed in the late 1980's with the more powerful turbofan engines, may be the reason that Airbus made the whole vertical stabilizer wider.

The 737 was originally designed in the 1960s to use the JT8D turbofan, not a turbojet.  The change that happened in the 1980s for narrowbody jetliners was from low-bypass to high-bypass turbofans; the change from turbojets to turbofans occurred in the early 1960s.


text:
What is this device below the door of a 737 (and other aircraft)?
The other day I made a question about the External Power Hatch. Now I would want to know what's that hole below the door.

That is the outflow valve. The outflow valve allows air to leave the pressurized section of the fuselage. The aircraft's pressurization system controls the valve to keep the desired pressure, balancing the air coming in through the bleed system.

Source

Outflow valve, controls cabin pressurisation. To pressurise the cabin, close this valve. Pump air into the cabin, stop the air flowing out, cabin pressurises. The valve doesn't completely close, but is about 98% closed. You still need some airflow going through the cabin. The cabin is pressurised to give an approximate equivalent altitude of 8,000 ft, whilst the aircraft is flying at 30,000 ft+.

Looks like the potable water supply point.

Image from 737 Airport planning document

No ... Potable water is on the captain's side, while the outflow valve - referenced in the OP - is on the FO side.


text:
Why do some cargo aircraft have windows?
There are some cargo aircraft that have windows which are not covered. Look:

On the second deck above where it says "Hong Kong Asia's World City" the aircraft has 3 uncovered windows. Source: Airliners.net
Also here:

Source: Airliners.net
On the upper deck of the Martinair 747 the windows are not covered. My question is: if the aircraft is used for cargo, why do they leave the windows uncovered?

There is still a crew that goes with the flight, including a load officer and possibly a relief crew.

I see a bird strike in the top image.

Both aircraft in your pictures are 747's. It's not really feasible to load cargo in the upper deck. The most comfortable jumpsuit I ever had on a FedEx freighter was the old 747-200 we had which was a converted passenger plane. The upper deck still had the first class seats installed.

Some cargo aircraft do have limited seating, as mentioned in the comments it could be for the relief crew, load officer, or other employees making a hop on the plane. Here is a nice blog post about  flying in a cargo plane. This article covers it pretty well also.

(source)
Some cargo planes are also old passenger planes and some of the windows may be left in place.

"Some cargo planes are also old passenger planes and some of the windows may be left in place. " - you can see in the OP's second picture that all of the windows in the lower deck and some of the ones on the upper deck have been painted over.


text:
Why are propellers de-iced before the engines are started, and does ice on the fuselage affect the flight characteristics of the plane?
I recently flew on a flight out of Montreal on a DHC8 turboprop.  The plane had been parked at the airport overnight during a winter storm, and as I walked out to the plane, there were visible icicles accumulated on the wings.  There was also a layer of ice completely covering the windows on one side of the plane, presumably because of the direction of the prevailing wind during the storm.
The plane was successfully de-iced and took off without a problem.  However, I had some questions about the de-icing process as I observed it from inside the plane.

The ground crew had to de-ice the propellers manually before the engines could be started.  Is this mainly because of the risk of ice ingestion in the engines, or is there a problem with the propeller being imbalanced because of uneven weight?  Or both?  Or neither?
The de-icing by the trucks seemed to focus on the wings, and it looked like there was still some ice left on the exterior windows when we left the de-icing pad. My understanding of icing is that ice on the wings is extra-bad (I remember a fatal crash happened near me due to wing ice when I was growing up), but does ice on the fuselage affect the flight characteristics of the plane?  Or is it not really a concern?

Ice on fuselage can bring down even an airship, it does not have to spoil any lift at all if it is heavy enough.

I've been in a plane where ice formed on the propellers during flight (a Dash7 (I think) from Thunder Bay to Fort Francis Ontario).  Eventually, enough ice formed that the spinning of the propellers caused the ice to separate from the propellers and be flung against the fuselage.  Given the noise it caused, I had to think it was bad from the plane.

@Flydog57 It was noisy but probably not bad for the plane.  The props are heated - the manufacturer knows ice will form and then be melted/thrown off.  Some turboprops even have armor plates on the fuselage along the plane of the props for this reason.

@nexus_2006: Yeah, I realize that.  But, boy, it was scary.  I was sitting right in the ice's "target zone".  It sounded like someone was shooting at the side of the plane with a large caliber rifle.  Together with the turbulence (from the storm that was causing the ice accumulation), it was the wildest flight of my life.  I don't miss the job I had in the 1980s that had me flying to Canadian paper mill towns.

The props are done before starting because you need to make sure the blades and spinners are fully cleaned off while they are stationary. Otherwise, they'd vibrate like hell when starting and shed bits of ice all over. Also, if you just sprayed the props while running the engines would ingest a lot of glycol, which, if it doesn't make the engine flame out, can gum things up and cause cabin smells as cooked fluid makes its way into the air conditioning system through the compressor bleed system (a lot of turn-backs from "cabin smoke" reports on departure are from the stink from cooked glycol getting into the cabin via the bleeds).
Even done stationary, some fluid will make its way into the engine and it's typical to run the engines at a moderate power setting after the treatment for a couple minutes to try to purge whatever glycol residue is in the compressor before opening the bleeds.
Fuselages are deiced depending on the airplane and the conditions.  On some jets, especially those with tail mounted engines, the upper fuselage is treated as if it was a flying surface and must be clear of ice and frost, even small amounts (it varies with the regulatory jurisdiction).  Otherwise it will depend on whether there is any significant accumulation, mostly for weight considerations.
If there is an obvious layer of snow/ice on the fuselage, it will be removed... mostly; don't expect a perfect job and they won't normally shoot down as far as the windows. If it's just morning frost, the fuselage may be left alone although the wings and tail must still be done (ANY frost on leading edges can be fatal).  The Capt is in communication with the deicing personnel working the boom and will give instructions on what is to be covered, and what to avoid (like APU air inlet in the tail, or heat exchanger inlets, etc.), whether to use Type I only or Type I and II/III/IV (Type one is for melting/removal and II/III/IV is for follow-on residual protection for a limited time).
A deice with follow on residual anti-ice protection costs the airline a couple thousand bucks per flight so avoiding unnecessary fluid application is important in the long run.

The only answer which actually answers the question "why are the props de-iced before the engines are started"

Ice flying off the propellers can damage something, or someone. Also unbalances the propeller assembly overall, leading to vibrations.
Little bit of ice left on the fuselage is not bad, only causes some extra drag until it sublimates off, and of course a little extra weight.  Ice Will impact the lift created by the wings, that is very bad, unless the ice is along the wing/fuselage joint, where it disrupt smooth airflow, altho probably just creating some more drag until it sublimates off.

Where you wrote "Will impact the lift created by the wings," did you instead mean to write, "[Ice on the fuselage] will not impact the lift created by the wings?"

Ice on airplanes causes two main problems.

It adds weight and drag.
It changes the shape of the aerodynamic surfaces.

Problem 1 is fairly obvious. How serious a problem it is depends on the airplane and how fully loaded it is. If it is a problem, then ice needs to be removed from any or all surfaces to reduce the problem to an acceptable level. If the plane is sufficiently over-powered or under its maximum gross weight, then this may be ignored.
Problem 2 is the main reason for deicing. The shape of the wings, propellers, and tail are carefully designed to achieve the desired airplane performance. (It may not be obvious, but propellers have an airfoil shape just as wings do.) Any change in shape is inevitably detrimental. If the shape of the propellers is affected by ice, then engine performance will suffer. If the shape of the wings is affected by ice, then lift will suffer. If the shape of the tail is affected by ice, stability and maneuverability will suffer.

This explains why icing is dangerous. It doesn't address why propellers are deiced first.

authors: Vladimir F Героям слава


text:
Which speed shall be used on high speed taxiway?
After landing, which speed shall we use to exit from high speed taxiway? Max. 60 knots ground speed or indicated air speed?

Depends on what you are flying. In a Cessna 150, 60 knots means you are low-altitude flying, not taxiing...

You may restrict your question to certain types of aircraft in certain categories of airport (e.g. airliners on international airports)

Possible duplicate of What is the maximum taxi speed and who defines it?

According to the ICAO manual on Aerodrome Standards (based on Annex 14) (link) - maximum 35 knots for code 1 and 2 runways, and maximum 50 knots for code 3 and 4 runways.
Since you are moving on the ground, it makes sense to use your ground speed as reference.
I imagine most aircraft operators have procedures for using rapid exit taxiways in their standard operating manuals. As a result, the actual speed used by air crews is likely to vary depending on which company they fly for.
IFALPA recommends that you slow down to normal taxi speed on the runway, and then use rapid exit taxiways with normal taxi speed (as opposed to the very low speed you would need to use to turn off the runway using a pendicular taxiway). (link)
From the ICAO manual:

3.8.15
A rapid exit taxiway shall be designed with a radius of turn-off curve of at least:

550 m where the code number is 3 or 4; and
275 m where the code number is 1 or 2;

to enable exit speeds under wet conditions of:

93 km/h where the code number is 3 or 4; and
65 km/h where the code number is 1 or 2.

Note: you can find the definitions of the code numbers (1, 2, 3, 4) on page 17 of the manual to which I linked above.


text:
Is there a formula for calculating the pivot point of an aircraft while turning on the ground?
I'm working on a program that requires a taxiing passenger aircraft (of any model) to perform a semi-accurate turn given a steering angle of the front landing gear. For this I need to calculate the pivot/center point for the aircraft to rotate around. There seem to be a few factors that influence this (including some basic specifications of the aircraft) but I can't seem to find even a general equation that breaks it down.
I'm aware that there are also a number of more complex factors that would influence this, but for my purposes I'm assuming an ideal situation with minimal interference.
Thanks in advance!

I think the easiest way to approximate this is to draw an imaginary line between the two rear gear. When the front wheel is turned 90 degrees left, the left tire is the pivot point. As the front tire straightens to 0 degrees, the pivot point linearly moves to the center of the line. Repeat as the front gear turns right.

If you assume the main gear does not rotate (in the left-right sense), then the pivot point will always be somewhere on a line joining the two main gears.
You can then use trigonometry and the turn angle of the nose wheel to identify the centre of rotation. (Assume the nose tyre does not slip sideways.)
(By the way this is very similar to Ackermann steering used on cars - lots of standard mechanical texts talk about this.)

Ah, seems simple enough. So, past my original question - if the main gear does rotate, how does this influence the center of rotation? What about wheel slip?

The main gear does not rotate significantly in the left/right direction. Like the rear wheels of a car, they just point forwards all the time and don't steer - so the centre of rotation must be on the same line. Also, if the tyres slipped sideways appreciably, the tyres would scuff at an appreciable rate.

Also, consider that many larger passenger/freighter craft have more than one MLG on each side.(747, A380, etc).

How does the distance change (adjacent leg d in TomMcW's diagram) with a more complex wheel arrangement? I'm guessing that for an aircraft with multiple sets of main landing gears, d extends to the half-way point between all main gears (and the pivot would be on a line that passes through that point). Or would the pivot be on a line that passes through a point between the main gears with the widest track? And when there are multiple bogey gears, I'm assuming from the middle of the gear?

The pivot point will be where an imaginary line through the rear wheels and an imaginary line perpendicular to the front wheel intersects.

So if θ is the steering angle and d is the distance between the front wheel and a point halfway between the rear wheels then
X = d • tan(θ)
will give you the distance from the point between the rear wheels to the pivot point.


text:
What exactly is a variable incidence tailplane?
I've been reading about it regarding trim changes and such, but I don't exactly know what a VIT is? Can someone explain that to me?

Incidence is the angle at which an aerodynamic surface is mounted to the remaining structure of an aircraft. Variable means that this angle can be changed in flight. Normally, a control surface is split between a forward part, called the stabilizer, and the rear part, called the elevator. On aircraft with less powerful wing flaps, the stabilizer incidence is fixed and only the elevator can move, while more complex aircraft can also move the stabilizer.
A third way is to do away with a separate elevator and to move the whole tail surface. This is a variable incidence tailplane.
Benefits of a full-flying tail surface

By avoiding a contour break due to a flap deflection, all-moving control surfaces can avoid the shocks which would otherwise occur at high subsonic speed. This is especially beneficial for supersonic aircraft.
At supersonic speed, the uncambered surface will produce less drag, so for supersonic speed the all-moving control surface is more efficient.
By moving all of the tail surface at high speed, an all-moving control surface will produce the highest rate of moment change over time possible. To produce also the highest sustained moment change, however, it needs to be larger than a comparable stabilizer-elevator combination because it forfeits the benefits of variable camber.
The all-moving control surface has no gaps which could add radar reflections, allowing for a more stealthy design and slightly reducing drag.
Lower mechanical complexity. This is somewhat offset by the need for much beefier hydraulics.

Some gliders have full-flying horizontal tail surfaces for reduced complexity. They have a slight back sweep to shift their aerodynamic center and their center of gravity to the same location. This makes the tail light and simple and avoids the added drag of the elevator hinge gap. However, such tail configurations have caused a number of crashes in the early phase of a winch launch when the pilot did not keep the stick neutral. At low speed a fully deflected tail surface has separated flow on the suction side, rendering the tail less effective. Only when the flow suddenly attaches as speed picks up the tail effectiveness is regained. In combination with the pitch-up moment of the tow line this lead to very steep angles of attack right after take-off which the pilot could not correct with the still ineffective tail, and some gliders stalled, rolled and crashed as a consequence. Therefore, modern gliders avoid this feature.

Schempp-Hirth Cirrus with full-flying tail (picture source)

It is basically what its name implies: an horizontal stabilizer with a variable angle of incidence (not to be confused with angle of attack). It might be used for trimming purposes only (like in most modern airliners and a few GA aircraft, control with the trimming system) or for pitch control (in most if not all supersonic, non-delta-winged fighters, controlled by pilot's input).
You can read more here: https://en.wikipedia.org/wiki/Stabilator (especially the Airliners section).


text:
Aerodynamics of a Propeller
I'm having troubling understanding where the relative wind is coming from for a propeller. The relative wind that I do know is the one that comes right in front of the aircraft which flows over the wings which is an airfoil. The propeller is also an airfoil, which leads me to believe the relative wind would come from the opposite direction of the propeller, but where?

"The propeller is also an airfoil": Correct, but this airfoil is not moving in the same direction than the aircraft. It's actual trajectory, relative to air, is approximately the vector addition of the horizontal move (same than wings) and the rotation of the blades. The vectors addition is shown on @Canuk 's answer and also in the duplicate. After you have the actual direction of the blade trajectory, then "the relative wind would come from the opposite direction" is indeed true.

The NOAA National Weather Service Glossary defines relative wind in a way that might help you make sense of this:

Relative Wind: The wind with reference to a moving point. Sometimes called APPARENT WIND.

From the diagram in the linked question, you can see that the relative wind is a vector opposite the propeller's 0 degree angle of attack. Simplified, this relative wind is the speed the wind would appear to flow if the air was moving, instead of the propeller. But since the propeller is moving and not the air, we call it relative wind: relative to the propeller, the air is flowing over it because the propeller is moving.
However, I'm sensing from your question that you might think there is only one relative wind vector, and that is the one parallel and opposite direction to the 0 degree angle attack of the main wings.  However, the as you can see from the definition, any moving point is going to have relative wind!
Remember that relative wind is just the wind in reference to a moving point. So once the airplane is in forward motion, even though the air in the propeller's plane of rotation is no longer still, the relative wind that the propeller "feels" is still coming from the same direction as before, but the magnitude may be modified by the change in the actual speed of the air in the propeller's plane of rotation. Then to get the strength of the relative wind on the propeller, you'd need to add the wind vector caused by the forward motion.
At the risk of over-explaining, remember it's relative wind. Rather than the air blowing (creating wind), the object is moving, so the wind is relative to the moving object.


text:
Adding moisture creating fog?
It may be a simple question and I've read the PHAK and searched online, but I can't seem to understand why adding moisture to a dry cold air mass would create fog. I understand when an air parcel is cooled to the dewpoint neared the surface, it will become saturated and fog appears. However, why would adding moisture create fog?

You may want to ask this on Earth Science.SE for a better reply.

I've re-closed this as a duplicate of our "How can you anticipate cloud formation" question because the concepts are fundamentally similar, but the answer below captures the gist of it.

You can either cool the air to reduce it's capacity to hold water, or you can add water to the air. Either way will eventually make the air saturated
You can either make the bucket smaller, or you can add water in the bucket. Either  way you will eventually make the bucket spill ...

When it's completely full and saturated, so would it be at its dewpoint?

Correct.  When temperature = dewpoint relative humidity will be 100%. At that point the water vapor will start to condense creating fog (or clouds if you're above the ground level.)  Add enough water and you'll raise the dewpoint to ambient temperature.  Or you can lower ambient temperature to the dewpoint and create fog without adding water.


text:
Why does MAP show ambient pressure when engine is shut off?
Manifold pressure is the absolute pressure at the intake manifold and increases as throttle is increased and vice versa. However, when the engines are shutoff, the manifold reads ambient pressure. My question is, why does it read ambient pressure when engines are shut off when the throttles at closed?

The intake manifold is where the air flows from outside into the engine, so obviously it must be open to the outside. So when everything is calm, the pressure quickly equalizes and the manifold pressure is equal to the ambient pressure.
The non-obvious thing is actually why it differs when the engine is running.
When the engine is running, it sucks air in. The volume of air per unit of time is proportional to its rotational speed: higher RPM means more strokes per minute and each stroke has the same displacement.
Now without any restriction in the intake manifold, the air would still enter at the ambient pressure (speaking of normally aspirated engine). And with fully open throttle, the restriction is small enough that the manifold pressure is indeed only a little below ambient.
It is only as you close the throttle that you create significant restriction, which reduces the manifold pressure.
But the reduction is still proportional to the engine RPM. With engine not running there is no suction, so the pressure can't decrease and the gauge still shows ambient pressure.
This dependence on the RPM also means that it is not actually good indicator of the power the engine produces. When you increase RPM by adjusting the mixture, the engine power will increase—because it takes more power to sustain higher RPM—but the manifold pressure will decrease—because there is more suction. The same also happens when you increase RPM by advancing propeller pitch selector.
What manifold pressure is good indicator of though, and the reason it exists, is the engine wear. The amount of air, and fuel, are proportional to the manifold pressure, and therefore so is the peek pressure when the air is compressed and the fuel burnt. And the higher the peek pressure, the more wear on the components. That is why there is absolute maximum you should never exceed and continuous power limit that you should only exceed for limited time on take-off or in emergency.
Note that the relations are further complicated in turbocharged engines where higher engine RPM also mean higher turbocharger RPM and that increases the pressure gain on the turbocharger that adds to the manifold pressure.

Image source
Because the manifold is connected to the ambient air via the air intake. The throttle does not hermetically seal against ambient pressure differential when closed, there is no design requirement for this condition.
The transducer measures pressure, not pressure differential.

@Koyovia Does the MP gauge read the uncorrected barometric pressure or the ATIS altimeter setting pressure? I’m guessing uncorrected.

@JScarry The altimeter setting reported by the ATIS is QNH (Pressure at sea level calculated from QFE (Pressure at field elevation) using the ISA (International Standard Atmosphere)-model). The MAP would indicate QFE (or at least close to) - to the extent that it is calibrated.


text:
Can some planes turn using just the rudder or without banking?
I know you use the rudder when banking to keep the plane in the correct orientation, but is it possible, when making a slight turn with say a small private cessna, could you just use the rudder to turn?

@ymb1, if you apply rudder and keep the ailerons neutral, the most planes will roll into the turn and will do that at any speed. It is by design.

Yes it is possible to turn without ailerons. It is not recommended though.
I have tried it on Cessna 172 as practice many times. I started practicing this maneuver some years ago after I witnessed a disabled Cessna with its ailerons jammed at an angle keeping it in perpetual turn was safely flown back to Van Nuys airport by an experienced instructor on the right seat. when you apply say left rudder the plane will bank left and turn left but you have to control the turn rate by power and rudder. The rate at which one applies the rudder is important; also you should be aware and anticipate potential risk of rolling.
Again it is not recommended but in case of emergency it could save the day!

Note that on A320 if you lose all flight computers (extremely unlikely, of course), you are left with just horizontal stabilizer and rudder control and the aircraft was demonstrated controllable with that.

And NASA also tested methods to control an aircraft with thrust only. Control with thrust was used both successfully (DHL Baghdad 2003) and not (UA 232 -- This story was depicted in a Mayday episode).


text:
How frequent are touch-and-gos?
Other questions on this site say that go-arounds are roughly 1-3 per 1,000 landings. But in many of them, as described by passengers, the wheels don't actually touch the ground. Recently I had a touch-and-go (at Nuka Hiva, a small airstrip on a cliffy island in the Pacific) where our wheels most definitely touched the ground but we immediately accelerated and went around. The brief announcement said it was due to wind shear - we were safely on the ground on the second try so all's well that ends well.
I'm curious as to whether a true touch-and-go when a pilot is not practicing touch-and-go, on a commercial flight, is a significant rarity. By coincidence, this very same flight started with my first ever aborted takeoff, because a fuel overfill on one side had left us unbalanced; we returned to the gate and span one prop for 10-15 minutes then took off without incident. Am I in lottery-ticket territory?

I'm not sure that you will ever get an answer since touch and gos are classified as go arounds.  I even remember reading about a go around once where the PF elected to put the wheels on the ground before climbing away to eliminate the chance of a tail strike since the decision to abort was made very late.  A really good reason would be a touch down beyond the TD zone with an assessment that there is insufficient runway remaining to safely stop.

I found this video showing x-wind landings.  There are a couple of touch and gos which might suggest that they are not rare.

In larger aircraft you may not be able to execute a go around once you have touched down but its not always because of weather. This mainly has to do with the fact that once a plane touches down (situation depending) it may have already committed to the landing and will no longer be able to get airborne with the remaining runway. There is a discussion here on the topic and an article here that covers it as well.
It generally takes more distance to take off than land and a considerable amount of speed is bleed off during the flair and subsequent touch down. If a plane touches down at the proper touch down point on a runway it may not have enough distance to accelerate and become airborne again by the other end of the runway. Further more you can complicate the situation with things like thrust reversers which, once deployed force you to commit to the landing.
The airport in question has a 5578 x 98 feet runway which is a solid size but chances are you were flying a plane on the smaller side. Generally the smaller the plane the less runway you need to land and take off so go arounds after touchdown are possible.
To directly answer your question, in some instances they do happen but they will be recorded as an aborted landing. In other cases (heavy crosswinds) you may have only one side of the mains touch (does that count as a touch down?). The runway length and aircraft size/performance are the main factors and there are so many combinations of that.
--Edit--
Since you have provided the make and model of the plane we can extrapolate a bit more. You can find the full specs here but lets take the the At TOW for 300 Nm - Max pax - 3,000 ft - ISA +10 - situation which is quoted at 4,626 ft. Take off speed at max load is 116 Kts (and you were landing so you were on the lower end of the fuel which means you were below max load even with full passengers). Likewise approach speed is 113 Kts. Some of this depends on the pilots precision but It should be (and obviously was) possible to touch down, full power, and get airborne again in enough distance on the strip. Sine you would only need to make up 3 Kts assuming he came in on the mark at 113. They would lose speed in the touch down but if they were on the throttles fast enough it would work.

it was indeed a small plane (though not the smallest of my trip) - an ATR 72 Turboprop.


text:
Is it possible to take checkride past AFSP training request expiration?
My AFSP training request expires next week. Can I do my instrument checkride past that date?
Technically checkride is not flight training (all of my training is complete at this point), so it's not clear to me if I can do it or not.

@mins I don't think this is a dupe, even if they come from the same scenario. As I understand them, the other question asks if an existing approval can be extended, whereas this one asks if an approval is even needed for a checkride.

Agreed, this asks a completely different question, and one that I am now curious about!

Yes, this is definitely a different question.

@Pondlife. Ok, VTC retracted.

As far as I can tell (and from my own experience), you don't need any TSA approval for a checkride.
Looking at the regulations, 49 CFR 1552.1 defines flight training as "instruction received from a flight school" (including from individual instructors). A checkride isn't instruction, as you said, although I can't find a regulation that explicitly says that (probably because there's no real need for one).
And looking at the FAA's requirements for checkrides:

The FAA's instructions to examiners for an instrument checkride don't mention security or say that they have to check anything related to the TSA or AFSP as part of the preliminaries (see section 5-434, Establishing Eligibility and 5-446, Procedures)
There's no mention of the TSA either in 61-65F - Certification: Pilots and Flight and Ground Instructors (apart from some notes on verifying identity)

There's also nothing in the IACRA process about the TSA or security either, which makes sense because that should match the checkride eligibility instructions.
My own experience is also that it isn't relevant to the checkride. I've done two instrument checkrides after getting the TSA's approval for training and neither DE asked me for any information about the TSA or AFSP, they were only interested in my logbook (and medical etc.). I suppose there could have been some extra check behind the scenes or in IACRA that I wasn't aware of (and my TSA approvals were still valid) but I doubt that was the case.
Finally, I realize that all this still doesn't give a positive confirmation that it's allowed, so if you want to have that I suggest you call the AFSP hotline and/or your local FSDO to get a definite statement.


text:
Aircraft CG limitations: what drives constraints beyond max weight & a single forward (and aft) limit?
In a chart like the one below, limitations on the max aircraft weight (upper horizontal line), forward-most CG (left vertical line), and aft-most CG (right vertical line) are fairly easy to understand. However, other limitations can be present as well, such as the diagonal line that cuts off what would be the top-left corner below, so as to disallow some forward CG's at heavier weights that would be okay when lighter.

Source
At a conceptual level, what drives limitations like this diagonal line? What adverse state does adhering to this limit protect against?

The additional offset of the cg must be balanced by additional negative lift of the elevator, which would decrease total lift.

The "aircraft C.G. location" is indeed the C.G. location of the loaded aircraft.
At some given (constant) speed for final approach, in the "cut off" upper left hand corner of the chart the aircraft runs out of elevator authority for the landing flare.
Related links:
https://www.pilotsofamerica.com/community/threads/weight-and-balance-gurus-enter-in.120993/

Also, stick forces may become excessive in a reversible control system.

The Piper Seminole light twin suffered from that problem, the pitch forces becoming unpleasantly high in the flare with only two aboard and trimmed for normal approach speed. You could cheat it by adding more nose up trim just before you were about to flare, but you left yourself exposed if you had to go around and now you were frantically retrimming ND.

When designing an aircraft -- specifically sizing the horizontal tail and elevator -- we use an 'X-Plot' or a 'Scissor Plot'.
This plots tail size (often as volume coefficient) on the Y vs. CG position on the X, with several slanted vertical lines, each representing a requirement for the tail.  These requirements later become these CG envelope limits.
Anything that might represent a limit of tail size or elevator size can become one of these limits.
Having sufficient control authority to rotate on takeoff.
Having sufficient control authority to stall on landing.
Having sufficient trim authority to trim throughout the flight envelope.
Some sort of nose-down control limit.
Aft-cg stability limit, fwd-cg stability limit.
Here is an example:

Pulled from this paper.
When sizing the tail, you can imagine taking a horizontal bar with the desired % CG range and dropping that into the gap between the constraints.  In the case of a 20% desired range, the tail area ratio in this plot is 0.23.  However, if we required a 30% CG range for some reason, we would need a larger tail - about 0.27.

Your plot and answer shows horizontal tail area vs. CG but the question is about weight vs. CG

Indeed -- typically the constraints are the same on both plots.  One is used during the aircraft design process.  The other is used operationally after the size of the tail is fixed.  I was trying to give a list of constraints that apply to the CG position -- as well as a bunch of keywords and context that may be searched for further research.

Then you should maybe better highlight the connection between the limitation on the weight and the limitation on the tailplane surface.


text:
Why do you pull back on the stick in a forward slip?
In a forward slip (I'm learning to fly sailplanes, if that matters), quite a lot of back pressure is needed to keep the nose up in a forward slip. Why, aerodynamically speaking, is this the case?
I understand that, to transition from coordinated flight to a forward slip, the aircraft effectively has to rotate around its yaw axis. From the perspective of the pilot, this results in a bit of bank angle and simultaneously raising the nose. However, in a fully developed forward slip I've often had to fully pull back on the stick to prevent the nose from dropping and gaining too much speed, which is normally not necessary to slightly raise the nose.
I asked an instructor, and they told me that due to the T-tail construction of a glider, the horizontal stabilizer sees extra pressure underneath it due to the relative wind blowing at the vertical stabilizer, pushing the tail up. However, in my reasoning, the other half of the elevator should see reduced pressure as it is shielded by the vertical stabilizer. Additionally, the horizontal stabilizer is simply less effective as its projected wingspan is less in a forward slip - but surely the main wings also lift proportionally less?
Can someone shed some more light on why (almost) full stick back is needed to maintain a forward slip in a sailplane?

On which sailplane did you experience this?

Re "Additionally, the horizontal stabilizer is simply less effective as its projected wingspan is less in a forward slip - but surely the main wings also lift proportionally less?" - I guess that's just another way to say that it you look at the angle between the relative wind and the longitudinal axis of the aircraft, in coordinated flight 100% of that angle is "projected" as the angle-of-attack, but in slipping flight some of that angle is "projected" as the a-o-a but some of that angle is "projected" as the yaw or slip angle.

I have some recollection of noticing that this tendency is particularly pronounced in a Schweizer 1-36, which is a T-tailed glider. It's been some years ago but my recollection is that I was playing around with contrlling the glider with rudder only, hands off the stick, and if the yaw/slip angle accidentally got too severe (for example while trying to pick up a lowered wing w/ rudder only) the nose would really want to plunge down, in a way that I hadn't seen w/ 1-26, Ka6, and some others.  (Not quite the same as obsrving that "almost full back stick is needed to maintn a fwd slip" though.)

I've so far slipped in an ASK21, LS4 and Duo Discus, I guess the ASK21 was most pronounced but the others required quite some back pressure too

@user721108 -- it would be, but how do you exactly measure the airspeed in a slip?  Do the test on a calm day and use your gps groundspeed?

@quietflyer I don't think the airspeed varies that much during a slip, usually when you pull out of it you're maybe a few kmh lower or higher than your target landing speed

@Sanchises -- yesterday I had the chance to refresh my memory as to the sideslip characteristics of a Schweizer 1-26 glider (not a t-tail configuration) -- a full-rudder non-turning slip ("forward slip", "sideslip", whatever, essentially they are all one and the same thing) produces no extreme changes in airspeed or pitch attitude, if you leave the stick in about the same fore-and-aft position as before beginning the rudder input.  Definitely no need for full aft stick, and you'd undoubtedly stall if you did that.  I'm sure the T-tail configuration one of the key parts to what you experienced.

@Sanchises -- funny that "Community" just bumped this to the top of the stack as yesterday had the chance to refresh my memory re sideslip characteristics in another glider, Libelle-- basically same as 1-26; actually I felt I needed to apply some gentle forward pressure on the stick to maintain the same-ish airspeed as I had trimmed for in non-slipping flight.  But there's so much "stiction" (static friction) in the linkages and the pitch control forces are so light that it's hard to make detailed observations re pitch trim characteristics in this glider--

A T-tail produces a noticeable pitch-down moment change in a sideslip.
That is basically all. You go into a steady sideslip and need to retrim to keep the speed at the desired setting. This will not be needed in gliders with a low mounted horizontal tail.

Thanks for the find, so this is actually a duplicate!

@Sanchises Maybe, but that fact is not mentioned in the major books about flight mechanics. Few professionals know about it.

Looking at the tail design of the Schweizer SGS 1-36 Sprite one might tend to believe the instructor and thank the designer.
Flying a forward slip involves intentionally cross controlling the aircraft in an effort to increase drag, which should steepen the glide angle at the same speed.
This does result, as the name slip implies, in a pronounced change in relative wind on the vertical stabilizer.  With the T-tail mount essentially acting as an end plate, it is understandable that increased pressure on the horizontal under surface will produce a downward pitch.
A secondary reason might be a disruption of the downwash sheet the wing produces by slipping, removing downward pressure on the tail.
Since a glider flying at low speeds is only a few knots above stall speed, and most glider pilots are loathe to waste energy on higher drag configurations (except for landing), it seems the tendency to have to add back pressure on the stick is arguably better than to allow airspeed to slow unacceptably.
However, this might be a case where spoilers may be preferred to extreme forward slipping.  A combination of spoilers and moderate slipping may be better.  Aerodynamic performance will vary between models. Expert instruction is invaluable.

During my first flights in a glider, there were some construction works that we were to overfly at 100m or more. The resulting final approach was always full spoilers and slipping. Quite a memorable way to start flying.